THE AIR-SACS AND THEIR USE 



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fel 



originating from the spiracles. All these large sacs are superficial, 

 lying directly beneath the hypodermis, while the smaller ones are 

 buried among the muscles. We have detected 53 of these vesicles 

 in the head. 



In the honey- 

 bee (Fig. 42(5) and 

 humble bee (Fig. 

 427) as well as the 

 flies there are two 

 enormous air-sacs 

 at the base of the 

 abdomen. In lar- 

 val and wingless 

 insects these sacs 

 are entirely absent. 



The use of the 

 air-sacs. It was 

 supposed by Hun- 

 ter as well as by 

 Newport, and the 

 view has been gen- 

 erally held, that 

 the use of these 

 sacs is to lighten 

 the weight, i.e. les- 

 sen the specific 

 gravity of the body 

 during flight. It 

 has, however, been 

 suggested to us by 

 A. A. Packard that 

 this view from the 

 standpoint of 

 physics is incor- 

 rect. It is evident 

 that the wings 

 have to support 

 just as much 



weight when the insect is flying, whether the trachese and vesicles 

 are filled with air or not, the body of the insect during flight not 

 being lightened by the air in the sacs. The use of these numerous 

 sacs, some of them very spacious, is to afford a greater supply of 



